Color Measurement – the CIE Color Space Chapter 9 Color Measurement – the CIE 1931 System
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BOOK THREE OF COLOR MANAGEMENT Color measurement – the CIE color space Chapter 9 Color measurement – the CIE 1931 system Introduction The measurement of a color is basically nothing other than standardised color vision, where the two factors, light and observer are standardised. The scientific foundation for color measurement is based on the existence of 3 different groups of signals (primary color stimuli blue, green and red) that are passed on from the eye of the observer. The starting point for transfer into a standardised system is the sensitivity of the S, M and L cones. Today, the sensitivities, related to the wavelength, are known. As a basis for an international colorimetric system, the International Commission on Illumination, the CIE (“Commission Internationale de l’Éclairage”) in 1931 defined three spectral colors as primary color stimuli – red R = 700.0 nm, green G = 546.1 nm and blue B = 435.8 nm. The sensitivity of the cones, however, is also dependent on the angle of observation. Standardisation was implemented using the CIE standard observer concept. The standard observer, like the standard illuminant type, is a table of numerical values that represent an “average standard human observer”, so the color perceptions are not specific to an individual observer. Luminous efficiency of the human eye – lightness In the visible range of the electromagnetic spectrum (400 nm - 700 nm), the human eye perceives the same spectral radiances to have different lightnesses at different wavelengths. This spectral luminous efficiency of the eye was measured and standardised by the CIE for the standard observer. The V() curve applies for all photopic vision, in which the cones in the retina are active. The Luminous efficiency of the eye values for the luminous efficiency for photopic vision were drawn up in 1923 by the CIE, and adopted in 1924 to carry out colorimetric calculations. 1,5 The V´() curve applies for scotopic vision, in which the rods are the active receptors. The luminous efficiency V’ V values for scotopic vision were gathered into a standard 1,0 by the CIE in 1951. In the luminous density range between photopic and scotopic vision – the mesopic range (twilight vision) – the spectral sensitivity curve shifts 0,5 with a decreasing adaptive luminous density, even for shorter wavelengths. Spectral radiated power Spectral radiated 0 400 450 500 550 600 650 700 750 Wavelength (nm) Luminous efficiency of the eye Curve V = photopic vision/day = (CIE 1924) Curve V’ = scotopic vision/night = (CIE 1951) 01 Datacolor | Color and color measurement The human observer's perception of color stimulus Experiments with normal-sighted human observers were The entire range of color stimuli perceivable by people carried out in order to define a “standard observer” as could be recorded in this way; color vision ability was the basis for all colorimetric measures and calculations. recorded numerically. A divided screen was used in these experiments. On The most significant experiments in determining the one side, one specific color was projected, and on trichromatic perception of color stimulus of the eye were the other side were three projectors in the light colors carried out in 1928 by W. D. Wright and 1931 by J. Guild. blue, green and red. The observer had to reconstruct The trials by Wright and Guild were able to demonstrate the color impression of the first color by changing the that the numerical values differed slightly from each other lightness of the three light sources (three color theory). because the primary light sources were slightly different. The radiation quantity was noted in a table for each These experiments in additive color mixing corroborated change to the three primary light sources and each Young's three-component theory. change to the experimental light at each wavelength. 1,4 Red 1,2 1,0 Green 0,8 0,6 0,4 Blue Experiment in the 0,2 Versuch der additiven Mischung von additive mixing of Black partition Lichtstrahlung 0 light radiation -0,2 Observer Light intensity Light -0,4 -0,6 R G B projection surfaceWhite -0,8 Test lamp Screen -1,0 400 450 500 550 600 650 700 Wavelength (nm) Chapter 09 | Color measurement – the CIE 1931 system 02 Chapter 9 The colorimetric 2° or small field CIE31 standard observer In the experiments on additive color mixing, it was shown On account of these restrictions, in 1931, the CIE that not all real colors could be generated with the defined three arbitrary imaginary measurement values (X, CIE's three RGB primary color stimuli. It was sometimes Y and Z) as primary color stimuli – selected from the point necessary to mix the color sample with one of the three of view of easy colorimetric evaluation. All real colors primary colors to reach equivalence with the mixture from can be represented by additive mixing using these three the two remaining primary colors. This means that certain measurement values. These measurement values are colors can only be mixed from the three primary colors called ‘CIE standard tristimulus values’, and the color if one primary color contributes a “negative portion”. For space is called ‘CIE XYZ color space’. some spectral colors, therefore, the colorimetric values Transformation of the RGB primary color stimuli into must be negative. the XYZ primary color stimuli was carried out with the following characteristics: Negative values in the equations were to be eliminated (negative values were extremely difficult to process ≠ G1+G2 electronically at that time) Definition of a new system with three ‘imaginary’ G1 G2 G3 primary color stimuli x, y, z, so the spectral locus falls 0 into a triangle that is defined by these three primary color stimuli The function y was selected and calculated to correspond with the luminous efficiency function V ( ) (CIE 1924), therefore simplifying the calculations The function z was equated with zero for most of the visible spectral range, also to simplify the calculations P P+G3 = G1+G2 The calculations were carried out for a light source with equal radiation as well as for the entire spectral range, G3 G1 G2 G3 so the areas of the x, y, z-functions are the same. 0 The resulting functions are called CIE-x, y, z-color-matching functions. They are not real functions in the proper sense; they represent the average standard observer. P P = G1+G2-G3 G1 G2 G3 0 z Negative primary color portion in color 2,0 matching by mixing the primary color with the color sample P. 1,5 y x 1,0 P + G3 = G1+ G2 density factorLuminous 0,5 is equivalent to 450 500 550 600 650 700 P = G1+ G2 - G3 0 400 Wavelength (nm) P ... Color sample, G1, G2, G3 ... three primary colors Color-matching functions x, y, z of the 2° standard observer (CIE31) 03 Datacolor | Color and color measurement The colorimetric 10° or large field CIE64 The field of vision of 2 ° standard standard observer observer corresponds to the size of a coin of 1 Euro that you hold with To allow human perception to be incorporated in a an outstretched arm in front of you. controlled way into a measurement result, it was necessary to define a standard for human vision. This standardised vision was defined by the CIE standard observers. The CIE31 standard observer or 2° standard observer is traced back to an experiment to determine the average color perception of the human observer, For this, it was 2° considered that humans perceive colors most precisely when they encounter the area in the eye where vision is sharpest (fovea, yellow spot). With a normal viewing distance from a color sample, this region deviates by about 2° from the optical axis of the eye. From this, it was determined that the angle at which the standard observer sees should be exactly 2°. This corresponds to a field of vision the size of a 1 euro coin that you hold in front of you with your arm outstretched. The normal field of vision of human perception, however, is greater than this 2° area. Jacobsen (1948) and Judd (1949) were also able to demonstrate that the colorimetric calculations based on the 2° angle did not correspond very well to the actual observations in the range of the short wavelengths (especially for violet). As a result, the CIE proposed another standard observation angle in 1960 10° – the 10° standard observer. This corresponds to a field of vision the size of an A4 page at a standard viewing distance of 30 cm. The color-matching functions x10, y10, z10 of this new standard observer were then ultimately specified by the CIE as standard in 1964. CIE 1931 2° standard observer CIE 1964 10° standard observer 2,0 x y z Color-matching functions z 10° 1,5 y 2° 1,0 x 0,5 Spectral radiated power Spectral radiated 0 400 450 500 550 600 650 The field of vision of 10 ° standard observer is Wavelength (nm) 700 about 27 times as large as the 2 ° standard observer. Color-matching functions x, y, z of the 10° standard observer (CIE 1964) Chapter 09 | Color measurement – the CIE 1931 system 04 Chapter 9 The chromaticity diagram according to the Example: Calculation of the standard tristimulus CIE standard colorimetric system CIE31 value X of a color specification. For each wavelength of the visible spectral range, the Using the standard color-matching functions x, y, z of the value of the color-matching function x is multiplied by standard observer, the spectral curve can be converted the value of the spectral radiated power S of a standard into 3 values – the standard tristimulus values X, Y and Z.